Processing techniques for wide-angle seismic data

238 Pages·2006·28.85 MB·English

Processing techniques for wide-angle seismic data

Hassan Masoomzadeh

A dissertation submitted for the degree of
Doctor of Philosophy
at the University of Cambridge

Wolfson College
Cambridge

September 2005
Copyright Hassan Masoomzadeh, 2005
Any individual is hereby authorized to copy all or part of this dissertation for private
study purpose, provided that this notice is included with any substantial portion copied,
that the headline and footline are included in any page reproduced by mechanical means,
and that full acknowledgement of the source is made for any material from the
dissertation used in published or publicly presented work.
Declaration

This dissertation describes my original work except where acknowledgement is made in
the text. It does not exceed the page limit and is not substantially the same as any work
that has been, or is being submitted to any other university for any degree, diploma or any
other qualification.

Hassan Masoomzadeh
September 2005
To my family
For their admirable patience throughout these years …
Acknowledgements
I would like to express my greatest gratefulness to my supervisors Penny Barton and
Satish Singh for their continuous supervision and incredible support during all the years
of my study in Cambridge. They kindly proof read all my written materials and wisely
instructed me towards the end. I also thank my academic friends Richard Hobbs and
James Jackson for their invaluable suggestions and assistance.
I am thankful for the help I received from my contemporary team mates in the LITHOS
research group. Miguel Bosch kindly helped me to get started when I arrived. A great
man with a big heart and an endless enthusiasm for helping, named Immo Trinks, assisted
me in many different aspects. He introduced me a number of software applications
available in Bullard Labs, he even spent time helping me to improve my English. Geraint
Jones kindly proof read a major part of my thesis and supplied many helpful comments.
Dhananjai Pandey, Timothy Sears and Anusha Surendra are also acknowledged for their
kind help. I never could appropriately thank Paula Aarons for her tremendous help in a
wide variety of kinds, started even before I arrive and going on still. Clare Enright and
David Lyness are also acknowledged for their prompt help with computing problems
whenever occurred.

A great acknowledgment goes to the National Iranian Oil Company (NIOC) and the Oil
Exploration Operation Company (OEOC) for providing an extended scholarship covering
my full time education and family life in Cambridge. In particular Dr. Moosavizadeh, Dr.
Haghighi, Dr. Attar, Mr. Nikpoor, Mrs. Taghizadeh, Mrs. Aghababie and the other
colleagues from the Petroleum University of Technology (PUT) are acknowledged for
facilitating my study abroad. LITHOS science group is also acknowledged for providing
financial support and sponsoring my language course and my attendance to the
international conferences.

The data sets used in this study were supplied by BGS Rockall consortium and the
STATOIL Company. I would like to thank Joseph Gallagher, Peter Dromgoole, Dave
Ellis, Even Brenne, and Robert Evans from STATOIL for their frequent visits,
enthusiastic supervision and helpful comments.

I would not have been able to finalize my course without continuous encouragement and
sympathetic support of my wife who literally sacrificed four long years of her early 30’s,
looking after three boys; me and the other younger ones!
Processing techniques for wide-angle seismic data

Hassan Masoomzadeh

A new processing strategy is implemented to enhance seismic images obtained from
wide-angle seismic data, acquired for sub-basalt imaging purposes. The conventional
sample-based processing approach may be altered to a coarser scale of a window-based
processing technique in which a reflector and its vicinity remain un-stretched during the
normal moveout (NMO) correction. Iso-moveout curves in the time-velocity panel are
introduced and employed to perform a constant moveout correction, leading to a horizon-
consistent multi-window non-stretch stack. Further improvements are achieved by data
processing in the τ-p domain, including downward continuation and shifted-elliptical
non-stretch dynamic correction.

Chapter 1 provides an introduction to problems associated with sub-basalt imaging using
wide-angle seismic data. In Chapter 2 the stretching caused by NMO correction and the
existing non-stretch techniques are reviewed, and a new method of non-stretch stacking
using iso-moveout curves is introduced. A zigzag velocity function is then introduced as
a combination of a number of local iso-moveout curves. Using the zigzag velocity
function the stretching is avoided and the high-frequency information at far offset is
preserved. The method is computationally cost effective because no mathematical
transformation of seismic data is required; only the stacking velocity field needs to be
modified horizon-consistently and then used in a standard NMO routine. In Chapter 3 the
argument is extended into the τ-p domain where the seismic wavefield appears different
and requires an alternative set of processing techniques. The basic processing approaches
are examined in the τ-p domain, including deconvolution, demultipling, velocity analysis,
dynamic correction, muting and stacking. Non-stretch stacking, fourth-order shifted-
elliptical dynamic correction and downward continuation in the τ-p domain are discussed
in detail.

In Chapters 4 and 5 the ideas discussed in Chapters 2 and 3 are applied to wide-angle data
sets from the north Atlantic margin. Non-stretch stacking, exploiting most of the
available arrivals, was able to provide convincing images consistent with the geology of
the investigated areas. The employed processing strategy facilitated exploitation of wide-
angle arrivals in order to provide enhanced images of the sub-basalt structure. The
stacking-velocity fields, obtained via an interactive animated analysis approach, were
converted into interval-velocity fields and used for imaging purposes. As concluded in
Chapter 6, a significant improvement is achieved for the data set possessing the longest
maximum offset (30 km) particularly for the zone of the basement interface. A moderate
improvement is gained for the data set with 12 km maximum offset at the zone of sub-
basalt reflectors, and a limited enhancement observed for the data set with 8 km
maximum offset in the zone of intra-basalt targets.
Contents

1 Introduction .......................................................................................... 1
1.1 Wide-angle seismic data ................................................................................ 1
1.2 Wide-angle data acquisition .......................................................................... 2
1.3 Wide-angle data analysis ............................................................................... 4
1.3.1 Moveout correction and stretching ..................................................... 7
1.3.2 Residual moveout ............................................................................... 9
1.4 Processing in the τ-p domain ....................................................................... 10
1.5 Sub-basalt imaging using wide-angle data ................................................ 13
1.6 Structure of the dissertation ....................................................................... 15
2 Non-stretch stacking ........................................................................... 17
2.1 Introduction .................................................................................................. 17
2.2 Travel time equation .................................................................................... 20
2.2.1 Long-offset travel time ..................................................................... 24
2.3 Normal moveout correction ........................................................................ 24
2.4 NMO stretching ............................................................................................ 26
2.4.1 Anatomy of stretching ...................................................................... 29
2.4.2 Stretching parameters ....................................................................... 33
2.5 Non-stretch moveout correction ................................................................. 37
2.5.1 A brief history ................................................................................... 37
2.5.2 Window-based moveout correction .................................................. 39
2.5.3 Window size and taper ...................................................................... 41
2.5.4 Horizon consistency .......................................................................... 43
2.6 Iso-moveout approach ................................................................................. 43
2.6.1 Iso-moveout curve ............................................................................ 43
2.6.2 Zigzag velocity function ................................................................... 49
2.7 Constant moveout scan ................................................................................ 50
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2.8 Discussion and conclusions ......................................................................... 51
3 Processing in the τ-p domain ............................................................. 53
3.1 Introduction .................................................................................................. 53
3.2 τ-p transform ................................................................................................ 54
3.3 Processing in the τ-p domain ....................................................................... 56
3.3.1 Applications of τ-p processing ......................................................... 57
3.4 Amplitudes in the τ-p domain ..................................................................... 58
3.5 Spiking deconvolution ................................................................................. 60
3.6 Predictive deconvolution ............................................................................. 61
3.7 2-D filtering ................................................................................................... 62
3.8 Velocity analysis ........................................................................................... 64
3.8.1 Refraction-free velocity analysis ...................................................... 64
3.8.2 Interval velocity estimation .............................................................. 66
3.9 Dynamic correction ..................................................................................... 67
3.9.1 Downward continuation ................................................................... 70
3.9.2 Shifted-elliptical correction .............................................................. 73
3.10 Non-stretch correction ................................................................................. 74
3.10.1 Iso-moveout approach ....................................................................... 76
3.10.2 Layer stripping approach .................................................................. 77
3.10.3 Dix conversion .................................................................................. 78
3.10.4 Non-stretch shifted-ellipse ................................................................ 81
3.11 Slowness optimization .................................................................................. 82
3.12 Stacking ......................................................................................................... 83
3.13 Conclusions ................................................................................................... 84
4 Application to a data set from the Rockall Trough ......................... 86
4.1 Introduction .................................................................................................. 86
4.1.1 The Rockall Trough .......................................................................... 86
4.1.2 The Rockall profile ........................................................................... 88
4.1.3 Previous work ................................................................................... 89
H. Masoomzadeh, Ph. D. Dissertation, 2005
ix
4.1.4 Processing strategy ........................................................................... 92
4.2 Deconvolution ............................................................................................... 94
4.3 Radon filtering ............................................................................................. 96
4.4 Velocity analysis ........................................................................................... 98
4.5 Iso-moveout analysis ................................................................................... 99
4.6 Non-stretch NMO ....................................................................................... 100
4.7 Offset optimization ..................................................................................... 104
4.7.1 Scanning over offset range ............................................................. 104
4.7.2 Scanning over stretch percentage ................................................... 108
4.7.3 Trace weighting .............................................................................. 111
4.8 Processing in the τ-p domain ..................................................................... 113
4.8.1 Higher-order dynamic correction ................................................... 114
4.8.2 Non-stretch processing ................................................................... 115
4.9 Window size and horizon-consistency ...................................................... 118
4.10 Final stack ................................................................................................... 118
4.11 Imaging ....................................................................................................... 121
4.11.1 Modelling ........................................................................................ 121
4.11.2 Migration ........................................................................................ 125
4.12 Conclusions ................................................................................................. 126
5 Application to data sets from the Faeroe-Shetland Basin ............ 128
5.1 Introduction ................................................................................................ 128
5.1.1 Faeroe-Shetland Basin .................................................................... 129
5.1.2 Processing problems ....................................................................... 130
5.1.3 Processing strategy ......................................................................... 131
5.2 Profile STO 107- 4015 ................................................................................ 133
5.2.1 Data preparation .............................................................................. 134
5.2.2 True amplitude recovery ................................................................. 134
5.2.3 Radon filtering ................................................................................ 138
5.2.4 Offset optimization ......................................................................... 141
5.2.5 τ-p transformation .......................................................................... 143
H. Masoomzadeh, Ph. D. Dissertation, 2005
x
5.2.6 Deconvolution in t-x and τ-p .......................................................... 148
5.2.7 Velocity analysis ............................................................................. 155
5.2.8 Conventional moveout correction ................................................... 156
5.2.9 Non-stretch moveout correction ..................................................... 157
5.2.10 Downward continuation .................................................................. 161
5.2.11 Slowness optimization .................................................................... 165
5.2.12 Converted-wave processing ............................................................ 167
5.3 Profile STO 308 .......................................................................................... 168
5.3.1 Data preparation .............................................................................. 169
5.3.2 Geometry assignment ..................................................................... 170
5.3.3 Merging CMP gathers .................................................................... 170
5.3.4 Demultiple ...................................................................................... 171
5.3.5 Spectral whitening .......................................................................... 175
5.3.6 Velocity analysis ............................................................................ 178
5.3.7 Conventional stacking .................................................................... 180
5.3.8 Non-hyperbolic constant moveout scan ......................................... 183
5.3.9 Non-elliptical constant moveout scan ............................................ 183
5.3.10 Horizon consistent non-stretch stack .............................................. 184
5.3.11 Interval-stacking velocity estimation ............................................. 188
5.3.12 Downward continuation ................................................................. 189
5.3.13 Post stack enhancement .................................................................. 189
5.3.14 Migration ........................................................................................ 190
5.4 Conclusions ................................................................................................ 193

6 Conclusions ....................................................................................... 196
6.1 Overview ..................................................................................................... 196
6.2 Future work ................................................................................................ 202
6.3 Summary …................................................................................................. 205

Appendix A ....................................................................................... 206
A.1 Divergence correction for long-offset data ........................................ 206
H. Masoomzadeh, Ph. D. Dissertation, 2005